The present application is based on Japanese application 2000-043017, filed on Feb. 21, 2000, and Japanese application 2001-002728, filed on Jan. 10, 2001 which are both hereby incorporated by reference in their entireties.
1. Field of the Invention
The present invention relates to a collector for alkaline secondary batteries including a plated nonwoven fabric, to a method for making the same, and to an alkaline secondary battery using the same.
2. Description of the Background
Alkaline secondary batteries, which are highly reliable and are suitable for a reduction in weight, are widely used as power sources for various devices and apparatuses from portable devices to industrial large facilities. In most alkaline secondary batteries, nickel electrodes are used as positive electrodes. A nickel electrode has a structure including a collector for collecting electricity and a positive-electrode active material inducing a cell reaction supported on the collector. As collectors in this case, a sintered nickel plate formed by sintering nickel powder and a punched nickel plate have been widely used. The cell capacity is determined by the volume of the active material loaded in pores in such a nickel plate, and the volume of the loaded active material depends on the porosity of the nickel plate. Thus, it is preferable that the porosity of the nickel plate be as large as possible.
However, in sintered nickel plates and punched nickel plates, the porosity is as low as 75% to 80%. Moreover, the nickel content in a nitrate solution is low. Thus, the loading cycle for impregnation and neutralization must be repeated several times in order to load a predetermined amount of active material. Since the penetration of the nitrate solution into the interior of the nickel plate is impaired as the loading cycle is repeated, high density loading of the active material is barely achieved. Recently, a collector with a three-dimensional network structure has been used in order to enhance loading density of the active material into the collector to meet the requirements for higher capacity of batteries, since this structure has large porosity and thus can has high loading density for the active material.
The collector having the three-dimensional network structure is generally fabricated as follows. A porous network structure, such as a polyurethane foam sheet or an organic nonwoven fabric is plated with nickel by a known process, and is fired in a reducing atmosphere to pyrolyze the polyurethane sheet or the fabric so that the plated nickel network skeleton remains. In the resulting collector, a portion for an external terminal is flattened, the pores are filled with an active material paste, and a small nickel piece as an external terminal is spot-welded to the flattened position. Since the resulting collector has large pores and the porosity is as large as 90 to 98%, pasted nickel hydroxide can be directly loaded into the pores with high loading density. This collector contributes to an increase in capacity of alkaline secondary batteries.
However, this three-dimensional network structure does not have strength required for the collector and is too rigid. Thus, producing an electrode using this collector and assembling the electrode into a battery cause the following problems. When an active material paste with high viscosity is loaded into the collector, the active material paste is injected from the surface into the internal pores of the collector under a predetermined pressure. After the loaded active material paste is dried, the collector is rolled to increase the density and to optimize the electrode thickness, and is cut into pieces with a predetermined size. When the pressure applied to the paste is increased to improve the loading density of the paste, the nickel network skeleton of the collector may buckle or chip. Thus, the pressure on the active material paste must be reduced to avoid such buckling or chipping. However, desirable loading density of the paste is not achieved under a low pressure.
Since the nickel itself constituting the network skeleton is rigid, the network skeleton will leave cracks and projections such as scuffing on the outer periphery of the electrode using this collector, with chipping of the network skeleton in many cases, during winding the collector with a separator in the assembly of a cylindrical storage battery. These projections increase the electrical resistance of the electrode and impair the function of the collector and charge/discharge characteristics of the battery. In a prismatic storage battery using this collector, the collector swells due to a change in volume of the active material during charge/discharge cycles in some cases. Hence, separation may occur between the collector and the active material, or in the active material, resulting in deterioration of charge/discharge characteristics due to deterioration of the collector itself.
In addition, this collector with three-dimensional network structure is produced by many complicated steps with low productivity and relatively high cost. Moreover, the metal, i.e., nickel, which is only the constituent of the collector, precludes a decrease in thickness or weight of the collector. Accordingly, this metallic collector does not sufficiently meet the requirements for a decrease in weight and size.
In order to overcome this problem, Japanese Unexamined Patent Application Publication No. 8-329956 discloses a collector having a three-dimensional network structure. In this collector, a polyurethane foam sheet or a polyolefin nonwoven fabric is plated with nickel so as to impart conductivity to only the surface of the sheet or nonwoven fabric without pyrolyzing the sheet or nonwoven fabric. This collector can be produced by simpler steps, is flexible, and has relatively high strength, in comparison with the above-mentioned pyrolyzed collector with a three-dimensional network structure. No crack or projection causing scuffing forms during winding an electrode using this substrate together with a separator to assemble a cylindrical or prismatic battery. This collector exhibits improved charge/discharge characteristics and can meet the requirement for a reduction in weight and size.
However, in this collector, adhesion is insufficient between the polyurethane foam sheet or polyolefin nonwoven fabric and the plated nickel. When this collector is used as a nickel electrode of a nickel-hydrogen battery, the collector does not have a satisfactory function in a combination with a nickel hydroxide active material. Thus, it is difficult to assemble high-capacity batteries.
Japanese Unexamined Patent Application Publication No. 5-290838 discloses a method for making a nonwoven fabric electrode in which the nonwoven fabric is corona-treated prior to nickel plating. The corona-treated nonwoven fabric exhibits higher bonding strength to the plated layer compared to untreated nonwoven fabrics.
However, in this method, the bonding strength between the base material and the plated layer is still insufficient in practice. When this collector is used as a nickel electrode in a nickel-hydrogen battery, the plated layer undergoes a change in quality or partial scaling during assembling a battery and repeated charge/discharge cycles of the battery. The resulting battery shows a short charge/discharge cycle life at high temperatures, resulting in an abrupt decrease in capacity.
Accordingly, it is an object of the present invention to provide a collector for an alkaline secondary battery exhibiting improved adhesiveness to plated nickel and a method for making the same.
It is another object of the present invention to provide an alkaline secondary battery which can be easily assembled and exhibits a high discharge rate and improved charge/discharge cycle characteristics.
According to a first aspect of the present invention, a collector for an alkaline secondary battery comprises a nonwoven fabric hydrophilized by sulfonation, gaseous fluorine treatment, or vinyl monomer grafting, and a nickel plating film formed on the nonwoven fabric.
The nonwoven fabric hydrophilized by the above treatment has a uniform and fine negative charge over the entire region. In this collector, the plated nickel film is tightly bonded to the nonwoven fabric, improving conductivity. Moreover, the plated nickel film does not scale off in use in an aqueous 20-35 weight % KOH solution, which is an electrolyte generally used in alkaline secondary batteries, over a long period, preventing an increase in surface resistance.
Preferably, in this alkaline secondary battery, the nonwoven fabric has a plurality of micropores extending from one surface to the other surface thereof. A large amount of active material is thereby loaded into the plurality of micropores, so that the alkaline secondary battery has high capacity.
Preferably, the diameter of the micropores is in the range of 0.1 to 5.0 mm, and the micropore density (the number of micropores per area) in the nonwoven fabric is in the range of 1 to 30/cm2. When the diameter is less than 0.1 mm or the micropore density is less than 1/cm2, a required amount of active material is not loaded. When the diameter exceeds 5.0 mm or the micropore density exceeds 30/cm2, the nonwoven fabric cannot maintain desired mechanical strength.
Preferably, the nonwoven fabric includes crimped fibers. Since the crimped fibers are bulky, the nonwoven fabric has an increased pore volume, which can load an increased amount of active material.
Preferably, the nonwoven fabric is produced by a wet process. The nonwoven fabric by the wet process is uniform with regard to weight and thickness, yielding a uniform electrode. Thus, an electrode with a uniform thickness can be formed using this collector. When this electrode is wound, an electrode group having high adhesiveness is formed, and a battery using the wound electrode exhibits superior charge/discharge characteristics.
According to a second aspect of the present invention, a method for making a collector for an alkaline secondary battery comprises a hydrophilizing step of a nonwoven fabric comprising at least one of a polyolefin fiber and a polyamide fiber, and a plating step of applying nickel plating to the hydrophilic nonwoven fabric.
In this method, the hydrophobic polyolefin or polyamide fiber, which precludes penetration of an aqueous plating solution and nickel plating, is made hydrophilic. Thus, nickel ions are firmly affixed to the surface of the nonwoven fabric in a nickel plating treatment, the plated nickel layer tightly bonded to the nonwoven fabric. The resulting collector exhibits high conductivity.
In this method, the hydrophilizing step preferably includes a treatment selected from sulfonation, gaseous fluorine treating, and vinyl monomer grafting. The nonwoven fabric hydrophilized by the above treatment has a uniform and fine negative charge over the entire region. In this collector, the plated nickel film is tightly bonded to the nonwoven fabric, improving its conductivity. Moreover, the plated nickel film does not scale off in use in an aqueous 20-35 weight % KOH solution, which is an electrolyte generally used in alkaline secondary batteries, over a long period, preventing an increase in surface resistance.
Preferably, in this method, the nonwoven fabric has a plurality of micropores extending from one surface to the other surface thereof.
Since an active material is also loaded into the plurality of micropores, a large amount of active material is loaded so that the alkaline secondary battery has high capacity.
In this method, the nickel plating is preferably electroless plating. The electroless plating facilitates the formation of a plated nickel film an the nonconductive nonwoven fabric.
Preferably, the method further comprises a step of forming an electroplating film by an electroplating process subsequent to the formation of an electroless plating film by the electroless plating.
Since the plated nickel film having a predetermined thickness is tightly bonded to the nonwoven fabric, the resulting collector has desired conductivity.
According to a third aspect of the present invention, an alkaline secondary battery comprises the collector according to the first aspect or a collector manufactured by the method according to the second aspect.
This alkaline secondary battery can be readily assembled using the collector according to the first aspect or the collector manufactured by the method according to the second aspect, and exhibits a, high discharge rate and improved charge/discharge cycle characteristics due to high adhesiveness of the collector to the plated nickel layer.